1. Field of the Invention
The present invention relates generally to a radio communication system, and more particularly to a radio communication transmitter providing multiple frequency bandwidths.
2. Description of the Related Art
Current digital radio communication systems are primarily designed to provide voice service, which can be allocated to one frequency bandwidth since voice service user information is unified and its information rate is fixed. In the next-generation digital radio communication system, however, the user information may vary, comprising data, video and multimedia services, in addition to voice service. Accordingly, in order to efficiently use the limited frequency bands available and expand user capacity, a new multiple frequency bandwidth system capable of efficiently using frequency resources is required, in which one system (e.g. a mobile and fixed radio unit) provides a plurality of frequency bandwidths, so that a narrower bandwidth can be allocated to a service using a lower information rate, and a wider bandwidth can be allocated to a service using a higher information rate.
In general, a digital radio communication system must provide multiple frequency bandwidth characteristics to provide data service having varying information and information rates, in addition to voice service. In particular, to provide adequate capacity and variable services in a radio communication system such as a direct sequence code division multiple access (DS-CDMA), it is necessary to provide a multiple frequency bandwidth system.
The construction of a transmitter capable of providing multiple bandwidths based on the unitary baseband digital radio communication system in accordance with the prior art is illustrated in FIG. 1. A source coding section 111 compresses sound, data or video information inputted by a user service. Channel coding sections 112 and 113 code the outputs of the source coding section 111 to minimize a bit transferring error during radio communication. Channel coding sections 112 and 113 each use a different channel coding mode, respectively; one channel coding section 112 is a convolutional coder, and the other channel coding section 113 is a turbo coder, the turbo coder being concatenated with a Read-Solomon coder and a convolutional coder. By using two channel coding sections 112, 113, a channel coding mode can be selected based on the user service and the required level of service quality. Accordingly, any one of the channel coding sections 112, 113 may be used in accordance with the user service and the required level of service quality. For example, in the case of voice service, channel coding section 112 consisting of only one convolutional coder is used, while in the case where a higher quality data service is required, channel coding section 113 being concatenated with the Read-Solomon coder and the convolutional coder is used.
A multiplexer 114 selects the outputs of the channel coding section 112 or the channel coding section 113 by using a control signal of a controller (not shown). A digital modulating section 115 digital modulates the outputs of the multiplexer 114 in accordance with the characteristics of a digital radio communication system. For example, in the case of the DS-CDMA, the digital modulating section 115 carries out a spectrum spread and a data modulation (such as binary or quadrature phase shift keying; BPSK/QPSK).
The output of the multiplexer is spread at any one of the specific bandwidths among the multiple bandwidths, depending on the type of user service, in the digital modulating section 115, and the information related with the spread spectrum is passed through a respective lowpass filter 116, 117, 118 by a switching section (not shown) in order to improve the efficiency of the bandwidth and reduce the inter symbol interference. In particular, in order to filter the different spread spectrum signals, the multiple bandwidth system utilizes a number of lowpass filters 116, 117, 118, with varying frequency bandwidths and operating speeds. Each lowpass filter consists of a digital finite impulse response filter to maximize the efficiency of the frequency bandwidth only, or a pulse shaping digital finite impulse response filter to maximize the efficiency of the frequency bandwidth and reduce the inter symbol interference. A root raised cosine type is most commonly used as the pulse shaping digital finite impulse response filter.
A digital/analog converter 119 converts each of the digital signals filtered from the lowpass filters 116, 117, 118 into analog signals. The digital/analog converter 119 must have a sampling speed capable of converting a signal spread at a maximum bandwidth among multiple bandwidths of the system into an analog signal. A radio circuit section 120 filters the outputs of the digital/analog converter 119 between an intermediate frequency bandwidth and a radio frequency bandwidth and amplifies and transmits the resultant outputs through an antenna.
The prior art, system as described above has several disadvantages. First, because a number of digital lowpass filters (or pulse shaping filters) are used to provide multiple frequency bandwidths, additional power is required by the system (a mobile or fixed radio system) and reducing the size of the system becomes more difficult; second, unnecessary additional power is required for fast sampling when sampling the signal spread at other than the maximum bandwidth; and third, the number of digital/analog converters required makes reducing the size of the system more difficult.
Accordingly, the present invention overcomes the disadvantages of the prior art by accomplishing the following two objectives. One object is to provide a transmitter capable of reducing power consumption and thereby reduce the size of a radio communication system providing multiple frequency bandwidths.
Another object of the present invention is to provide a transmitter capable of filtering spread spectrum signals of a multiple frequency system by using one lowpass filter, and variably controlling the sampling speed when converting the lowpass filtered digital signal into an analog signal.
In order to achieve the above objects, according to the present invention, a transmitter of a multiple frequency radio communication system providing a plurality of frequency bandwidths comprises: a clock generating section for receiving a predetermined bandwidth selecting control signal and then generating a clock having a speed in proportion to the predetermined bandwidth; a multiplexing section for inserting zero into a signal, spread modulated in a bandwidth selected according to the predetermined bandwidth selecting control signal, to produce an oversampled signal; a lowpass filter for receiving the clock and for lowpass filtering an output signal of the multiplexing section; a digital/analog converter for receiving the clock and then converting an output signal of the lowpass filter into an analog signal at the speed of the clock; and a switch for receiving the bandwidth selecting control signal and then switching an output of the digital/analog converter to a corresponding one of a plurality of intermediate frequency circuits according to the selected bandwidth.